In a typical optical information reproduction apparatus, an optical beam generated from a source is irradiated onto an information recording medium to produce a reflected optical beam, from which information is read out. To this end, there is provided a focus servo mechanism wherein a focus servo is operated based on an error signal extracted from the reflected optical beam, thereby rendering a focus of the optical beam to follow an information plane on the information recording medium. Additionally, a focus lead-in device is provided to initiate an operation of the focus servo.
Recently, the information recording medium has found increasingly diverse applications, e.g., compact disc (CD), digital versatile disc (DVD), blue-ray disc (BD) and the like and, accordingly, the optical information reproduction apparatus is also required to have a versatile capability of reproducing information recorded on such information recording media.
To meet such requirement, there has been developed an optical pickup for accessing, e.g., both a CD and a DVD, wherein one of two laser diodes is selected depending on the type of information recording medium to be accessed, the two laser diodes providing laser beams of two wavelengths, respectively, i.e., one for the CD and the other for the DVD. However, the information reproduction apparatus is incapable of identifying the type of the information recording medium instantly after the information recording medium is loaded therein; and, consequently, there may occur a mismatching between the type of the optical information recording medium and that of the optical beam used to access it: for instance, the optical beam having the wavelength intended for the DVD may be called upon to access the CD.
To alleviate the problem, therefore, a focus search may be performed before accessing the optical information recording medium such that the type of the information recording medium can be presumed based on the status of a beam reflected therefrom. However, a precise determination of the type of the information recording medium cannot be guaranteed due to, e.g., malfunctioning of the apparatus, surface irregularity or contamination of the media, or time limitation to make a correct decision, resulting in the aforementioned mismatching. Furthermore, in case the wavelength of the optical beam is not compatible with the information recording medium, generation of a desired focus error signal is not guaranteed even by performing the focus search, resulting in a failure of focus lead-in.
Further, in case of accessing, with one optical pickup, a plurality of information recording media, e.g., a CD and a DVD, each having a different depth from a surface thereof to an information recording plane, another problem may arise in that the object lens in the optical pickup may collide with the information recording medium since the mechanical moving range of the object lens may overlap with the range of a warping of the information recording medium and its surface oscillation, thereby causing a failure in the focus lead-in and damaging the object lens and the medium.
To overcome these problems, therefore, there has been proposed a method wherein the focus lead-in is performed only during the focus search in which the object lens is moving in a direction away from the information recording medium (see, e.g., Japanese Patent Laid-Open Application No. 2001-202632 (page 7, FIG. 3)). This related art method will now be explained in detail.
FIG. 7 shows a block diagram of the related art optical information reproduction apparatus. In FIG. 7, reference numeral 1 represents an information recording medium; reference numeral 2, an optical pickup; reference numeral 3, an object lens; reference numeral 6, a focus actuator; reference numeral 7, a focus error signal generation unit; reference numeral 9, a focus servo unit; reference numeral 16, a controller; reference numeral 17, an optical beam source; reference numeral 18, a focus searching unit; reference numeral 19, a servo converting switch; reference numeral 20, a focus servo lead-in unit; and reference numeral 21, a focus zero cross detection unit.
Hereinafter, operation of the conventional optical information reproduction apparatus having the structure described above will be described.
Optical pickup 2, having object lens 3, focus actuator 6 and optical beam source 17, irradiates an optical beam generated from optical beam source 17 onto information recording medium 1, and detects a reflected beam therefrom. Focus error signal generation unit 7 extracts a focus error signal from the reflected beam and provides the focus error signal to focus servo unit 9. Focus servo unit 9 then generates a focus servo signal from the focus error signal.
Focus search unit 18 generates a focus search signal used to control object lens 3 to approach or move away from information recording medium 1. Servo converting switch 19 selects one of the focus search signal and the focus servo signal in accordance with a command from focus lead-in unit 20 so as to provide it to focus actuator 6, such that focus actuator 6 moves the focus of the optical beam in a direction of an optical axis by operating object lens 3 in accordance with an operating voltage applied thereto.
When information recording medium 1 is loaded into the optical information reproduction apparatus, controller 16 performs a focus search wherein focus search unit 18 first generates a focus search signal, which is applied to focus actuator 6 via servo converting switch 19. Focus zero cross detection unit 21 detects a zero crossing of the focus error signal, which is obtained during the focus search, to generate a focus zero cross signal, which is delivered to controller 16. At this time, based on the focus zero cross signal, controller 16 determines that the focus of the optical beam arrives at an information plane of information recording medium 1, and sends an arrival command to focus servo lead-in unit 20. Focus servo lead-in unit 20 then performs a focus lead-in operation by controlling servo converting switch 19 to convert a signal applied to actuator 6 from the focus search signal to the focus servo signal, thereby operating the focus servo. However, as shown in FIG. 8, depending on the combination of the types of optical beam and information recording medium 1, the focus zero cross signal may be a fake signal, i.e., a focus error signal that zero-crosses at a point other than a just focus point. In this case, if the focus lead-in operation is performed in accordance with the fake signal, the focus servo will fail to initiate and, further, the information recording medium and the object lens may be damaged. Accordingly, the focus lead-in operation should be performed only when the focus search is executed on the condition that the fake signal be not generated, i.e., when the focus position of the optical beam move away from information recording medium 1. In this manner, initiation of the focus servo can be performed safely, to thereby avoid a damaging of information recording medium 1 and the object lens.
Referring to FIG. 9, the order of carrying out a proper focus lead-in operation will now be described. In FIG. 9, the focus position of the optical beam moves in a direction along the optical axis, in accordance with the focus search operation of focus actuator 6. The focus error signal which is detected, during the focus search operation, is shown in FIG. 9. First, the focus of the optical beam is made to move away from the information plane of information recording medium 1 by moving object lens 3 away from information recording medium 1. Then, the focus searching direction is reversed such that the focus of the optical beam approaches the information plane of information recording medium 1. At this time, based on a focus zero cross signal, i.e., an output of focus zero cross detection unit 21, it can be detected that the focus of the optical beam has crossed the focus zero cross point, although it is still not clear whether the focus zero cross signal is a fake signal or indicates a crossing of a just focus point. Thereafter, the focus of the optical beam passes beyond the information plane of information recording medium 1 for a predetermined time ΔT. Then, the focus search direction of focus actuator 6 is reversed, such that the focus of the optical beam crosses the information plane of information recording medium 1 and further moves away therefrom. When performing the focus search in that direction, the fake signal is not generated before the focus of the optical beam reaches the just focus point. Therefore, the focus servo can be stably initiated by performing the focus lead-in operation in accordance with the focus zero cross signal outputted from focus zero cross detection unit 21.
However, in such an optical information reproduction apparatus, since the focus lead-in operation is performed during the focus search in which the object lens moves away from the information recording medium, regardless of the wavelength of the optical beam source and the type of medium, there may arise such other problems that are discussed below.
There would occur no problems if the focus error signal without the fake signal or the focus error signal shown in FIG. 8 is generated; however, depending on the combination of the type of information recording medium 1 and the wavelength of the optical beam, the fake signal may be generated when the position of object lens 3 is closer to the information recording medium 1 than that generating the just focus point. In this case, the initiation of the focus servo would fail, damaging the information recording medium 1 and the object lens. These problems may easily occur, in case of a mismatch between the optical information recording medium and the type of optical beam, e.g., the CD being accessed with the optical beam of the wavelength intended for DVD or the DVD being accessed with the optical beam of the wavelength intended for CD, which may result from the lack of accurate information on the type of the information recording medium.
To further illustrate the problem, in case the optical pickup includes two types of optical beam sources accessing the DVD/CD based information recording media, respectively, the wavelengths of which are about 650 nm and about 780 nm, respectively, if the type of the information recording medium is matched with that of the optical beam, e.g., the beam for accessing the DVD is irradiated onto the DVD disc or the beam for accessing the CD is irradiated onto the CD disc, an ideal focus error signal without a fake signal can be obtained as shown in FIG. 2. Or, depending on the structure of the optical pickup, as shown in FIG. 8, a fake signal may be detected at a point far removed from the information recording medium. However, suppose the type of information recording medium is not compatible with that of the optical beam, e.g., the optical beam to access the DVD being irradiated onto the CD disc or the beam to access the CD being irradiated onto the DVD disc: for instance, in the former case, a fake signal might be detected at a point removed from the information recording medium, due to the oscillating profile of the focus error signal prior to reaching the just focus point, as shown in FIG. 3. Further, in the latter case, a fake signal might be generated at a point close to the information recording medium, due to the oscillating profile of the focus error signal after passing the just focus point, as shown in FIG. 5.
Therefore, even when the focus lead-in operation is performed during the focus search while the object lens is moving away from the information recording medium, if the focus servo is tried to be led in by using an optical beam which is not compatible with the information recording medium, there still exists a possibility that the initiation of the focus servo may fail due to the fake signal and, the information recording medium 1 or the object lens may be damaged.
Further, since the focus error signal, which is obtained when the type of the information recording medium is not compatible with that of the optical beam, is not originally intended to be used with the optical pickup, it is possible that such signals are not defined in the specification of the optical pickup. Therefore, no inspection thereof can be performed when manufacturing the optical pickup. Accordingly, even when the wavelength of the optical beam source is determined based upon the type of the information recording medium, which is presumed by performing the focus search right after loading the information recording medium into the optical information reproduction apparatus, a mismatching may still occur between the types of the information recording medium and the optical beam. In the worst case, the initiation of the focus servo may fail and further the information recording medium 1 and the object lens may be damaged.
On the other hand, in order to obtain an ideal focus error signal without a fake signal, if the focus lead-in operation is performed by executing the focus search in a direction moving away from information recording medium 1 after the focus of the optical beam crosses the just focus point, such would require a longer period of time in initiating the focus servo.
In contrast, if the just focus point is detected while the focus search is executed in a direction of moving the object lens toward the information recording medium, since the direction of the focus search is reversed only after a predetermined time, even if the focus of the optical beam has passed the just focus point, the object lens may approach too close to the information recording medium and, in a worst case, collide with it, if the speed of the focus search becomes too fast.
On the other hand, if the focus search speed is adjusted to be slow to avoid the collision, the time required to initiate the focus servo would be not only extended but also the speed of the focus search would not render it possible to follow an up-and-down movement of the information plane of the information recording medium caused by, e.g., a surface irregularity of the information recording medium, thereby failing a proper focus lead-in operation and increasing the risk of initiation failure of the focus servo.